It is generally believed that there are two types of stem cells in the bone marrow, i.e. hematopoietic stem cells (HSCs) and mesenchymal (MSCs) and that their repertoire of differentiation/reconstituting potentials are distinct and separate from each other. HSCs have been shown to produce blood cells and some cells in the tissues such as mast cells and osteoclasts. MSCs are thought to be responsible for a number of mesenchymal cells including fibroblasts, adipocytes, chondrocytes and osteocytes. Recently, studies have begun to question the distinction between the potentials of HSCs and MSCs. For example, transplantation of 3000 side population (SP) cells that are highly enriched for HSCs generated osteoblasts in vivo. Also in a transplantation study using retrovirally transduced bone marrow cells, a close relationship was suggested by a common retroviral integration site in clonogenic hematopoietic cells and osteoprogenitors from each of the recipient mice. In our laboratory, by transplanting clonal cells derived from single HSCs, we have documented that a number of tissue fibroblasts/myofibroblasts such as glomerular mesangial cells, brain microglial cells, fibroblasts in cardiac valve and tumor-associated fibroblasts are derived from HSCs. We have also documented that fibroblast colony-forming units (CFU-F), which had been thought to be progenitors for mesenchymal cells, are also derived from HSCs. Most recently, we succeeded in generating adipocytes in vitro from the bone marrow cells of mice that had been clonally engrafted with single HSCs and directly from single hematopoietic progenitors. We also made a preliminary observation of osteocytes in vivo in the bones of mice that had been clonally engrafted for 10 months. Together, these observations suggest that some mesenchymal cell types are not derived solely from the MSC. In this application, we propose to investigate the potential of the HSC to give rise to fibroblasts, adipocytes, osteocytes and chondrocytes in vitro and in vivo under both normal and pathological conditions. Project Narrative: Studies described in this application are likely to elucidate hitherto unknown reconstitution potentials of bone marrow hematopoietic stem cells, i.e. abilities to generate fat tissues, bones and cartilages. The knowledge gained from these studies will allow subsequent development of new, unique avenues of therapy for obesity and diseases of bones and joints.